This invention relates to a novel read head assembly employable, for example, in a hard disk magnetic storage system. A major objective of the present invention is to integrate a magneto-resistive read head with an inductive write head so as to provide improved resolution for reading high density recordings.
Magnetic transducer heads are widely used for reading from and writing to magnetic storage media such as fixed disks, removable floppy and rigid disks, and magnetic recording tape. Inductive heads have been widely employed to perform both read and write functions. Reading with an inductive element involves converting flux changes in the media into electrical signals which are output from the read/write head.
The strength of the read output signal from an inductive head is proportional to track width, bit length and media velocity relative to the head. Progress with magnetic media has affected each of the parameters in a manner adverse to the output strength from an inductive head. The development of magnetic media with higher coercivity has permitted greater storage densities. To take advantage of this increased storage capacity, track widths and bit lengths are diminished. In addition, disk-shaped storage media have become more compact, with popular form factors evolving from 14" to 8" to 5.25", and recently to 3.5" iameters. The smaller diameters have generally resulted in decreased linear velocity of the media relative to the head. This trend has reached a point where the flux available in the media can be insufficient to provide a reliable read output from an inductive head.
Inductive read heads are considered passive in the sense that they rely primarily on flux transitions in the media as the source of energy for the read output signal. Active read heads, in contrast, use the flux transitions to modulate a current or other signal carrier. Thus, signal output is not directly limited by flux strength. The stronger output available is easier to detect and less vulnerable to noise.
Magneto-resistive read heads, one class of active read heads, provide roughly an order of magnitude improvement in signal output when compared with inductive read heads. This makes them better suited for the narrow track widths and short bit lengths of dense storage media. In addition, magneto-resistive read heads are sensitive to flux level rather than to flux change rate, to which inductive read heads respond. Therefore, magneto-resistive heads are better suited for reading when the lower linear velocities of modern compact media are used. In short, magneto-resistive heads are superior to inductive read heads in reading information densely stored on compact, high-coercivity media.
The write function has not been subject to a parallel development. Working in the opposite direction relative to the read function, the write function converts electrical signals from a host system into flux transition in the media. Electrical signal strength has not proved to be a limiting factor in inductive writing.
On the other hand, magneto-resistive write heads have not proved practicable. The basic principle of a magneto-resistive head is that flux changes are converted to changes in resistance in a magneto-resistive sensor. This resistance is detected as a voltage change in an electrical path containing the magneto-resistive sensor as current flows through the path. For this conductor to generate the magnetic fields required for writing, it would have to be too thick to provide the sensitivity required by the magneto-resistive effect during reading. Hence, inductive heads are preferred for writing while magneto-resistive heads are preferred for reading compact high-density media.
Typically, a magneto-resistive sensor is a strip of magneto-resistive material which has a preferred alignment for a magnetic moment referred to as an "easy axis". The axis in the plane of the sensor orthogonal to the easy axis is the "hard axis". For the read head to respond linearly to flux levels in an adjacent medium, the current through the magneto-resistive sensor must be oblique to its magnetic moment. However, the easy axis of a magneto-resistive sensor tends to lie along its length, which defines a favored direction for current flow. Since the easy axis is a favored direction for both the current and the magnetic moment, transverse biasing is used to provide a linear operating region about the zero media field condition.
Four schemes for applying such a transverse bias are disclosed in "Magnetics of Small Magnetoresistive Sensors" by Ching Tsang, J. Appl. Phys. 55 (6), Mar. 15, 1984, pp. 2226-2231. The first scheme is shunt biasing in which current is passed through a conductor sensor adjacent to the magneto-resistive sensor; the magnetic effect of this current biases the magneto-resistive head. In a second scheme, soft-film biasing, a soft magnetic film is placed adjacent to the magneto-resistive sensor; the bias current through the magneto-resistive sensor magnetizes the soft film, which in turn applies a magnetic field to bias the magneto-resistive sensor. Hard film biasing has also been considered in which a permanently magnetized sensor is placed adjacent to the magneto-resistive sensor; the operative principle is then comparable to the soft-film scheme. While in the foregoing scheme the magnetic moment is rotated relative to the easy axis, in a canted current or "barber pole" biasing scheme, slanted conductor sensors force current to flow obliquely to the easy axis.
In a fifth "mutual" bias scheme, disclosed in U.S. Pat. No. 3,860,965 to Voegeli, two magneto-resistive sensors are magneto-statically coupled to bias each other. In particular, the current in one sensor generates the field used to bias the other, and vice versa, in a manner related to the shunt biasing approach. Thus, the current through each magneto-resistive sensor serves as both a sense current and a bias current. An output differential read signal is obtained from the two magneto-resistive sensors. This has the advantage of doubling the signal output while rejecting common mode noise.
One approach to providing an integrated inductive write and magneto-resistive read head includes distinct, adjacent read and write gaps. This approach is disclosed in "An integrated Magnetoresistive Read, Inductive Write High Sensitivity Recording Head" by C. H. Bajorek, S. Krongelb, L. T. Romankiw and D. A. Thompson, Magnetism and Magnetic Materials--1974, American Institute of Physics Conference Proceedings, No. 24, Ed. C. D. Graham, Jr. G. H. Lander and J. J. Rhyne. This integrated head consists of one turn vertical shielded magneto-resistive head directly on top of an inductive head. A center magnetic layer serves both as a shield for the magneto-resistive head and as a pole tip for the inductive head. The magneto-resistive head uses a permanent magnet biasing scheme. The sharing of this center magnetic layer provides a manufacturing advantage in that fewer lithographic processing steps are needed than are required for separate heads.
Further processing advantages can be obtained by positioning the magneto-resistive sensor between the pole tips of an inductive head so that both pole tips serve as shields for the read head. This obviates the need for separate shields for the magneto-resistive sensor. This approach is disclosed by J. C. v. Lier, G. J. Koel, W. J. v. Gestel, L. Postman, J. T. Gerkema, F. W. Gorter and W. F. Druyvesteyn in "Combined Thin Film Magnetoresistive Read, Inductive Write Head", IEEE Transactions on Magnetics, Vol. MAG-12, No. 6, November 1976, pp. 716-718. Lier et al. disclosed a barber pole bias scheme for the read head which was situated in an inductive write head.
The Bajorek et al. and Lier et al. references represent advances in integrating inductive writing and magneto-resistive reading. The advantages of integration include reduction of processing steps and other manufacturing and operation savings. However, each technological advance, while achieving certain objectives, introduces new obstacles which must be identified and overcome. The present invention is based on the discovery of an important source of performance limitations in integrated inductive write and magneto-resistive read heads, which limitations are then overcome.